It is a requirement for automobiles to be equipped with a system to provide a measurement of the speed of the vehicle. Early forms of vehicle speed measurement systems or ‘speedometers’ were largely mechanical and involved the use of a mechanism linking the rotation of an axle or a wheel of the vehicle to angular movement of a needle mounted in a calibrated gauge to provide a readout of vehicle speed. Modern speedometers, however, have moved from mechanical to electronic-based systems and typically involve a rotation sensor, such as an optical, inductive or magnetic sensor, mounted in the transmission or a wheel of the vehicle.
Common to these types of vehicle speed sensing techniques is that they rely on the rotation of the wheels/driveshaft providing an accurate indication of the speed of the vehicle over the ground. However, in some vehicle applications, and under some driving conditions, the wheel speed can be misleading for determining vehicle speed. This is particularly true with off-road vehicles, or road vehicles with off-road capability, since they may operate in conditions where considerable wheel slip occurs.
It has been proposed to use a Doppler velocity technique to provide a ground speed measurement for a vehicle. Such a system is known for tractors used in the agricultural industry, and such a system is also exemplified in EP0534418B1. In such systems a radar transceiver directs a radar beam onto the ground at an oblique angle at a predetermined frequency which is then reflected off the ground back to the transceiver. Due to the fact that the vehicle is moving relative to the ground, a frequency shift occurs between the outbound and inbound radar signals due to the Doppler effect. Since the frequency shift is directly proportional to speed, it can be used to determine accurately the absolute vehicle speed over the ground. However, up to now the practical application of such systems have been limited.
It is against this background that the invention has been devised.